I've had something similar happen. Have you oiled your machine? Change your needle. To me, it sounds like your upper tension. I will be quilting for a while and everything was working well, to have my stitches start getting funky. The top thread will look like it is lying on top and not penitrating into the fabric. It drives me crazy, but oiling the machine, changing the needle, rethread your machine and working on a scrap quilt sandwich to get the tension back to acceptable levels, usually get me there. When you adjust your tension is your presser foot down? I had never heard of such, but my sewing machine repair guy (this was a mechanical sewing machine vs. computer sewing machine)said that the tension 'takes' when the presser foot is down. Otherwise you are spinning the dial. Finally if all fails take your machine in to your dealer or sewing machine repair guy and have a tune up.